SCHEDULE 13G

Amendment No. 1
Peabody Energy Corporation
Common Stock
Cusip #704549104


Cusip #704549104
Item 1:	Reporting Person - FMR Corp.
Item 4:	Delaware
Item 5:	1,133,807
Item 6:	0
Item 7:	6,975,737
Item 8:	0
Item 9:	6,975,737
Item 11:	10.909%
Item 12:	    HC


Cusip #704549104
Item 1:	Reporting Person - Edward C. Johnson 3d
Item 4:	United States of America
Item 5:	30,300
Item 6:	0
Item 7:	6,975,737
Item 8:	0
Item 9:	6,975,737
Item 11:	10.909%
Item 12:	IN


Cusip #704549104
Item 1:	Reporting Person - Abigail P. Johnson
Item 4:	United States of America
Item 5:	0
Item 6:	0
Item 7:	6,975,737
Item 8:	0
Item 9:	6,975,737
Item 11:	10.909%
Item 12:	    IN



	SCHEDULE 13G - TO BE INCLUDED IN
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)




Item 1(a).	Name of Issuer:

		Peabody Energy Corporation

Item 1(b).	Name of Issuer's Principal Executive Offices:

		701 Market Street
		St. Louis, MO  63101-1826


Item 2(a).	Name of Person Filing:

		FMR Corp.

Item 2(b).	Address or Principal Business Office or, if None,
Residence:

		82 Devonshire Street, Boston,
Massachusetts  02109

Item 2(c).	Citizenship:

		Not applicable

Item 2(d).	Title of Class of Securities:

		Common Stock

Item 2(e).	CUSIP Number:

		704549104

Item 3.	This statement is filed pursuant to Rule 13d-1(b) or 13d-2(b)
and the person filing, FMR Corp., is a parent holding company
in accordance with Section 240.13d-1(b)(ii)(G).  (Note:  See
Item 7).

Item 4.	Ownership

	(a)	Amount Beneficially Owned:	6,975,737

	(b)	Percent of Class:	10.909%

	(c)	Number of shares as to which such
person has:

	(i)	sole power to vote or to direct
the vote:	1,133,807

	(ii)	shared power to vote or to
direct the vote:	0

	(iii)	sole power to dispose or to
direct the disposition of:	6,975,737

	(iv)	shared power to dispose or to
direct the disposition of:	0



Item 5.	Ownership of Five Percent or Less of a Class.

	Not applicable.

Item 6.	Ownership of More than Five Percent on Behalf of Another
Person.

	Various persons have the right to receive or the power to
direct the receipt of dividends from, or the proceeds from the
sale of, the Common Stock of Peabody Energy Corporation.
No one person's interest in the Common Stock of Peabody
Energy Corporation is more than five percent of the total
outstanding Common Stock.

Item 7.	Identification and Classification of the Subsidiary Which
Acquired the Security Being Reported on By the Parent
Holding Company.

	See attached Exhibit(s) A, B, and C.

Item 8.	Identification and Classification of Members of
the Group.

	Not Applicable. See attached Exhibit A.

Item 9.	Notice of Dissolution of Group.

	Not applicable.

Item 10.	Certification.

	By signing below I certify that, to the best of my knowledge
and belief, the securities referred to above were acquired in the
ordinary course of business and were not acquired for the
purpose of and do not have the effect of changing or
influencing the control of the issuer of such securities and
were not acquired in connection with or as a participant in any
transaction having such purpose or effect.

Signature

	After reasonable inquiry and to the best of my knowledge and
belief, I certify that the information set forth in this Schedule
13G in connection with FMR Corp.'s beneficial ownership of
the Common Stock of Peabody Energy Corporation at May
31, 2004 is true, complete and correct.

June 10, 2004
Date

/s/Eric D. Roiter
Signature

Eric D. Roiter
Duly authorized under Power of Attorney
dated December 30, 1997 by and on behalf
of FMR Corp. and its direct and indirect
subsidiaries


	SCHEDULE 13G - TO BE INCLUDED IN
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)



	Pursuant to the instructions in Item 7 of Schedule
13G, Fidelity Management & Research Company
("Fidelity"), 82 Devonshire Street, Boston, Massachusetts
02109, a wholly-owned subsidiary of FMR Corp. and an
investment adviser registered under Section 203 of the
Investment Advisers Act of 1940, is the beneficial owner of
5,840,897 shares or 9.134% of the Common Stock
outstanding of Peabody Energy Corporation  ("the
Company") as a result of acting as investment adviser to
various investment companies registered under Section 8 of
the Investment Company Act of 1940.

	Edward C. Johnson 3d, FMR Corp., through its
control of Fidelity, and the funds each has sole power to
dispose of the 5,840,897 shares owned by the Funds.

	Neither FMR Corp. nor Edward C. Johnson 3d,
Chairman of FMR Corp., has the sole power to vote or direct
the voting of the shares owned directly by the Fidelity Funds,
which power resides with the Funds' Boards of Trustees.
Fidelity carries out the voting of the shares under written
guidelines established by the Funds' Boards of Trustees.

	Fidelity Management Trust Company, 82 Devonshire
Street, Boston, Massachusetts 02109, a wholly-owned
subsidiary of FMR Corp. and a bank as defined in Section
3(a)(6) of the Securities Exchange Act of 1934, is the
beneficial owner of  987,540 shares or 1.544% of the
Common Stock outstanding of the Company as a result of its
serving as investment manager of the institutional account(s).

	Edward C. Johnson 3d and FMR Corp., through its
control of Fidelity Management Trust Company, each has
sole dispositive power over 987,540 shares and sole power to
vote or to direct the voting of 974,940 shares, and no power
to vote or to direct the voting of 12,600 shares of Common
Stock owned by the institutional account(s) as reported
above.

	Strategic Advisers, Inc., 82 Devonshire Street,
Boston, MA 02109, a wholly-owned subsidiary of FMR
Corp. and an investment adviser registered under Section 203
of the Investment Advisers Act of 1940, provides investment
advisory services to individuals.  As such, FMR Corp.'s
beneficial ownership includes 89,100 shares, or 0.139%, of
the Common Stock stock outstanding of Peabody Energy
Corporation, beneficially owned through Strategic Advisers,
Inc.

	Members of the Edward C. Johnson 3d family are the
predominant owners of Class B shares of common stock of
FMR Corp., representing approximately 49% of the voting
power of FMR Corp.  Mr. Johnson 3d owns 12.0% and
Abigail Johnson owns 24.5% of the aggregate outstanding
voting stock of FMR Corp.  Mr. Johnson 3d is Chairman of
FMR Corp. and Abigail P. Johnson is a Director of FMR
Corp.  The Johnson family group and all other Class B
shareholders have entered into a shareholders' voting
agreement under which all Class B shares will be voted in
accordance with the majority vote of Class B shares.
Accordingly, through their ownership of voting common
stock and the execution of the shareholders' voting
agreement, members of the Johnson family may be deemed,
under the Investment Company Act of 1940, to form a
controlling group with respect to FMR Corp.

	Edward C. Johnson 3d has sole voting and dispositive
power over 30,300 shares, shared voting and dispositive
power over 0 shares, and no voting or dispositive power over
0 shares.

	Fidelity International Limited, Pembroke Hall, 42
Crowlane, Hamilton, Bermuda, and various foreign-based
subsidiaries provide investment advisory and management
services to a number of non-U.S. investment companies (the
"International Funds") and certain institutional investors.
Fidelity International Limited is the beneficial owner of
27,900 shares or 0.044% of the Common Stock outstanding
of the Company. Additional information with respect to the
beneficial ownership of Fidelity International Limited is
shown on Exhibit B.




	SCHEDULE 13G - TO BE INCLUDED IN
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)

	Pursuant to instructions in Item 7 of Schedule 13G, this
Exhibit has been prepared  to identify Fidelity International
Limited, Pembroke Hall, 42 Crow Lane, Hamilton, Bermuda,
a Bermudan joint stock company incorporated for an
unlimited duration by private act of the Bermuda Legislature
(FIL) and an investment adviser to various investment
companies (the "International Funds") and certain institutional
investors, as a beneficial owner of the 27,900 shares or
0.044% of the Common Stock outstanding of Peabody Energy
Corporation.

	Prior to June 30, 1980, FIL was a majority-owned
subsidiary of Fidelity Management & Research Company
(Fidelity), a wholly-owned subsidiary of FMR Corp.  On that
date, the shares of FIL held by Fidelity were distributed, as a
dividend,  to the shareholders of FMR Corp.  FIL currently
operates as an entity independent of FMR Corp. and Fidelity.
The International Funds and FIL's other clients, with the
exception of Fidelity and an affiliated company of Fidelity, are
non-U.S. entities.

	A partnership controlled by Edward C. Johnson 3d and
members of his family owns shares of FIL voting stock with
the right to cast approximately 39.89% of the total votes which
may be cast by all holders of FIL voting stock.  Mr. Johnson
3d is Chairman of FMR Corp. and FIL.  FMR Corp. and FIL
are separate and independent corporate entities, and their
Boards of Directors are generally composed of different
individuals.  Other than when one serves as a sub adviser to
the other, their investment decisions are made independently,
and their clients are generally different organizations.

	FMR Corp. and FIL are of the view that they are not
acting as a "group" for purposes of Section 13(d) under the
Securities Exchange Act of 1934 (the "1934" Act) and that
they are not otherwise required to attribute to each other the
"beneficial ownership" of securities "beneficially owned" by
the other corporation within the meaning of Rule 13d-3
promulgated under the 1934 Act.  Therefore, they are of the
view that the shares held by the other corporation need not be
aggregated for purposes of Section 13(d). However, FMR
Corp. is making this filing on a voluntary basis as if all of the
shares are beneficially owned by FMR Corp. and FIL on a
joint basis.

	FIL may continue to have the International Funds or
other accounts purchase shares subject to a number of factors,
including, among others, the availability of shares for sale at
what FIL considers to be reasonable prices and other
investment opportunities that may be available to the
International Funds.

	FIL intends to review continuously the equity position
of the International Funds and other accounts in the Company.
Depending upon its future evaluations of the business and
prospects of the Company and upon other developments,
including, but not limited to, general economic and business
conditions and money market and stock market conditions,
FIL may determine to cease making additional purchases of
shares or to increase or decrease the equity interest in the
Company by acquiring additional shares, or by disposing of all
or a portion of the shares.

	FIL does not have a present plan or proposal which
relates to or would result in (i) an extraordinary corporate
transaction, such as a merger, reorganization, liquidation, or
sale or transfer of a material amount of assets involving the
Company or any of its subsidiaries, (ii) any change in the
Company's present Board of Directors or management, (iii)
any material changes in the Company's present capitalization
or dividend policy or any other material change in the
Company's business or corporate structure, (iv) any change in
the Company's charter or by-laws, or (v) the Company's
common stock becoming eligible for termination of its
registration pursuant to Section 12(g)(4) of the 1934 Act.

	FIL  has sole power to vote and the sole power to
dispose of  27,900  shares.


 	SCHEDULE 13G - TO BE INCLUDED IN
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)
	RULE 13d-1(f)(1)  AGREEMENT

	The undersigned persons, on June 10, 2004, agree and
consent to the joint filing on their behalf of this Schedule 13G
in connection with their beneficial ownership of the Common
Stock of Peabody Energy Corporation at May 31, 2004.

	FMR Corp.

	By /s/ Eric D. Roiter
	Eric D. Roiter
	Duly authorized under Power of Attorney
	dated December 30, 1997, by and on behalf
	of FMR Corp. and its direct and indirect
	subsidiaries

	Edward C. Johnson 3d

	By /s/ Eric D. Roiter
	Eric D. Roiter
	Duly authorized under Power of Attorney
	dated December 30, 1997, by and on behalf
	of Edward C. Johnson 3d

	Abigail P. Johnson

	By /s/ Eric D. Roiter
	Eric D. Roiter
	Duly authorized under Power of Attorney
	dated December 30, 1997, by and on behalf
	of Abigail P. Johnson

	Fidelity Management & Research Company

	By /s/ Eric D. Roiter
	Eric D. Roiter
	Senior V.P. and General Counsel